1. Field of the Invention
This invention relates to a take-out tension control apparatus for controlling the take-out tension in a film or sheet being taken out from a stretching unit in a film or sheet production line by a take-out roller and also to a take-up tension apparatus for controlling the take-up tension in the stretched film or sheet as the film or sheet is wound into a roll on a take-up reel.
2. Related Art Statement
In the stretching aparatus of a stretched film or sheet production line, a plurality of pairs of clips are provided at predetermined intervals along the opposite edges of the film- or sheet-like synthetic resin work being stretched. These clip pairs grasp the opposite edges of the work as it proceeds and the distance between the claip pairs is gradually increased as the clips guide the work. In this way, the work is stretched into a film or sheet having a predetermined thickness as it is driven at a predetermined line speed in the direction of travel. The stretched film or sheet obtained in this way is taken out of the stretching apparatus by a take-out roller and is then fed to a take-up unit to be wound on a take-up reel. In the conventional arrangement, the take-out roller is rotated at the same peripheral speed as the line speed in the stretching apparatus. The take-up unit for winding up the stretched film or sheet taken out from the stretching apparatus is usually provided with a transmitted torque control means, e.g., a magnetic powder clutch, provided between a pay-off roller drive motor and the take-up reel and controls the torque transmitted to the take-up reel to thereby control the take-up tension in the stretched film or sheet that is wound on the take-up reel.
The opposite edges of the film or sheet being stretched as it proceeds through the stretching unit are grasped and restricted by the clips. However, the central portion of the film or sheet running through the space is not directly restricted from the outside. When the film or sheet in this state is taken out by the take-out roller, the film or sheet in the stretching unit is subject to the influence of the take-out tension because the stretching unit does not have a function of shutting out the take-out tension produced by the take-out roller.
If the take-out tension is proper, the film or sheet can be rolled uniformly over its entire width without distortion or thickness fluctuations in the width direction. When the take-out tension is excessive, however, the film or sheet tends to be stretched, to a greater extent toward the central portion as shown in FIG. 1. On the other hand, when the take-out tension is insufficient, the film or sheet tends to be stretched to a greater extent toward the edges as shown in FIG. 2.
When a film or sheet stretched non-uniformly is once wound into a roll and then rewound into a plurality of narrower rolls while being longitudinally slit, the rewound rolls are apt to exhibit thickness fluctuations and local sagging, e.g. edge sagging or center sagging. If the degree of the fluctuation and/or sagging is pronounced, the stretched film or sheet will not be suitable as a commercial product. Further, the individual rolls produced by slitting and rewinding a non-uniformly stretched film or sheet are apt to suffer wrinkling and non-uniform distribution of tension so that it is difficult to obtain a high quality roll. It is found that the quality of the rolled film or sheet is influenced not only by the performance of the stretching unit but also by the take-out tension applied by the take-out roller to the stretched film or sheet issuing from the stretching unit. The take-out tension, therefore, has to be controlled to a proper level in order to improve the quality of the stretched film or sheet.
The prior art take-out roller, however, is rotated at a peripheral speed equal to the line speed in the stretching unit and does not have any tension control function. In other words, control for assuring uniform thickness of the stretched film or sheet is undertaken only in the stretching unit. Such control is insufficient for obtaining a stretched film or sheet of high quality.
Moreover, the proper take-out tension varies with the prescribed thickness of the film or sheet to be stretched by the stretching unit. Further, when the take-out roller driven for rotation at a fixed speed, the difference between the line speed and the peripheral speed of the take-out roller directly leads to elongation or contraction of the film or sheet in the direction of travel. Therefore, even though the difference may be very small, it will greatly effect the tension in the film or sheet.
For this reason, even when used in combination with a stretching unit providing excellent performance, a prior art in-line winder lacking take-out tension control capability cannot sufficiently cope with the thickness fluctuations and local sagging of the film or sheet, so that the number of rejectable products is large.
Further, there has recently arisen a need to manufacture very thin films with thicknesses ranging from several tens of microns down to several microns. Such very thin films are stretched with low tension, which makes it even more difficult to stretch these thin films uniformly in the width direction. Therefore, thickness fluctuations and local distortion of the film are produced, causing local sagging of the film and other such defects and reducing the product yield.
In many cases, the defects inherent in the film taken up with the in-line winder appear as sagging or the like only in the ensuing slitting and rewinding process. This results not only in a large number of rejects but also in the loss of the time and labor spent for rewinding.
Furthermore, the film or sheet taken out from the stretching unit has a large width, and this wide film or sheet must be taken up into a large diameter roll. Such a large diameter roll has a large weight. Further, the recent trend is toward producing very thin stretched films with the stretching unit and these films are also taken up as large diameter rolls. For example, when a polyester film with a thickness of 12 microns and a width of 6 m is wound into a roll with an outer diameter of 1.2 m, the weight of this roll is approximately 6 tons.
However, the smaller the thickness of the film or sheet, the lower must be the take-up tension. The elongation of the film or sheet caused at the time of winding due to the take-up tension will cause shrinkage of the roll after winding. This will produce a permanent strain in the film or sheet and is a cause for rejected products. The take-up tension in the film or sheet, therefore, is desirably as low as possible so far as such shape defects as irregular roll ends are not produced. For example, when winding a 12-micron thick, 6-meter wide film as noted above, the take-up tension is preferably controlled to be about 10 kg or less over the entire width. If the radius of the roll at this time is 0.6 m, the take-up torque may be very low, e.g., 6 kgm.
It is thus necessary to wind the film or sheet into a roll which is as heavy as 6 tons and has a great momentum with a very low take-up tension of about 10 kg.
Where such a heavy roll of film or sheet is produced with low take-up tension, the momentum of the roll being produced and the frictional loss of the required large-size take-up reel drive mechanism greatly effect the required take-up reel torque. Therefore, smooth and accurate control of the low take-up tension in the film or sheet can not obtained through control of the take-up reel torque.
For the above reasons, the prior art take-up apparatus of the type where the take-up tension is controlled through control of the take-up reel torque is not able to take up a film or sheet into a heavy roll with a low take-up tension and with a sufficiently high winding quality. This is especially true in the case of the very thin films being produced currently.